This invention relates to a blend polymer composition which is obtained by blending a fluorine-containing copolymer with polyvinylidene fluoride and meets a general desire to improve the impact resistance of polyvinylidene fluoride.
Among conventional fluorohydrocarbon resins, polyvinylidene fluoride (abbreviated to PVDF) has excellent mechanical and electrical properties and is very stable to corrosive chemicals and also to ultraviolet rays and radioactive rays and, besides, is superior in workability. Accordingly PVDF has wide applications and is largely used for laminating or coating metallic materials, for covering electric wires and cables and for molding machine elements such as valves besides general uses in the form of film, sheet or pipe.
PVDF is a crystalline resin, and some favorable physical properties of this resin are attributed to the high degree of crystallinity. On the other hand, PVDF lacks elasticity by reason of high degree of crystallinity and therefore has some shortcomings. For example, PVDF coverings on electric wires tend to spontaneously crack during storage, and PVDF sheets formed by extrusion or drawing are liable to be relatively low in tear strength because of the occurrence of significant molecular orientation during the sheet-forming process. Furthermore, in respect of impact resistance PVDF can hardly be said to be better than other fluorohydrocarbon resins. It is often experienced that the existence of small scratches or other defects on the surfaces of molded articles of PVDF becomes the cause of breaking of the articles by shocks of relatively small magnitude.
Some proposals have been made to obtain PVDF base resins which are less crystalline than PVDF itself and more elastic than PVDF by copolymerization of vinylidene fluoride (abbrevitated to VDF) with another suitable monomer or by blending PVDF with a plasticizer and/or a different kind of synthetic resin. However, the attempts to substantially lessen the crystallinity of PVDF by copolymerization have not been fully successful mainly because the resultant copolymers become considerably lower in melting point or softening point as the modulus of elasticity is lowered, so that restrictions are placed on the use of the copolymers at high temperatures. Besides, in many cases the copolymers are not comparable to PVDF in workability. The attempts to obtain a good blend of PVDF with either a plasticizer or a relatively soft resin have encountered a problem that the available plasticizers and resins are mostly poor in compatibility with PVDF, so that it is difficult to obtain a blend which is macroscopically homogeneous and retains favorable physical properties of PVDF.
It was reported that blending of PVDF with polymethyl acrylate, which is good in compatibility with PVDF, gives a blend polymer which is fairly soft and elastic and excellent in drawability. (Japanese patent application publication No. 55-35042 (1980)). However, in such a blend polymer the improvement in the elasticity is inevitably accompanied by degradation of chemical resistance and weatherability, which is very high in the case of PVDF, since the content of fluorine in the blend decreases as the proportion of polymethyl acrylate is increased. Also blending PVDF with a fluorine-containing polymer, which is called a fluorine rubber or a fluoroelastomer, was considered. Actually, however, such blending has not given good results because conventional fluoroelastomers are generally unsatisfactory in compatibility with PVDF so that in many cases it is difficult to blend a sufficient amount of fluoroelastomer with PVDF, and the blend polymers are liable to become locally heterogeneous and, therefore, inferior in mechanical properties.